My comments are in parentheses.
A few years ago when my great uncle passed away I was the one who inherited his huge castle like mansion. (Take out the word had.)
This dwelling (maybe use a different word here, it sounds awkward) was much like the castles you see in storybooks. It was tall and gray, with a huge iron gate and (you repeated with twice) steel narrow spikes preventing burglars from climbing over.
Behind the ancient exterior (I changed that), the inside of the castle was complete with motion detectors and bulletproof windows which(grammar) had been glazed several times over to prevent people from peeking in.
My uncle was a rather paranoid man despite his calm, wrinkly exterior. (I like this sentence a lot.)
His castle was not like the ones where Cinderella and Briar Rose and her fairies lived in.When you saw this mansion you would remember the story of the Selfish Giant and his Garden or, rather, (change comma place) without the garden because my uncle never believed in wasting money on unreasonable extravagance on his temporary accommodation, which is ironic because he spent over a fourth of his fortune securing his fortress and had been living in it for 20 years.(Maybe split this sentence into two...it's a little long.)
So, I wasn’t very eager to live in this house. I (myself isn't needed) preferred large spacious houses with large windows, near the beach or a park. Homey and fragrant (maybe this is a better word choice?), with a vase of daisies I had picked outside, not fake looking, clipped roses. This new house I had inherited shouldn’t even be considered a house, my uncle wouldn’t have permitted it, it was too much of a concrete/steel jungle with (grammar) straight box lines to make it comfortable enough to be a home. But I can not go against my uncle’s last will and testament, so in a few months I was standing in front of the door of the castle. (This is a really good last sentence, creates suspense.)
I really like the story so far, I would rate it 8. :)